Interaction of the components of the prothrombinase complex

The interaction of components of the prothrombinase complex, i.e. bovine Factor X or Factor Xa. bovine Factor V or Factor Va, phospholipid, and Ca²⁺, in various combinations was studied primary by a gel filtration technique. In experiments, in which phospholipids ranging from those isolated from naturally occurring sources to those long chain (18 : 1) as well as short chain 6 : o and 7 : 0 fatty acids prepared by chemical and enzymatic synthesis were used, it was evident that a net negative surface charge on the lipid dispersions was one of the important requirements for interaction. Though the short chain fatty acid phospholipids interacted with the proteins of the prothrombinase complex, there was invariably a diminution in the activity of the enxyme complex. It was established that Factor V or Va did not bind Ca²⁺ and that the binding of either of these factors with phospholipids (with a net negative charge) was not dependent on Ca²⁺. However, the interaction of Factor X or Factor Xa with phospholipids with a negative charge required Ca²⁺. It was shown that Factor X could bind to the same type of lipid surface as that notes for Factor Xa. Of interest was the apparent difference in the phospholipid binding characteristics of the two variant forms of bovine plasma Factor X, i.e. X₁ and X₂, which might in part explain the differences in their specific activities. Of importance was the lack of demonstrable complex formation between Factors II, X and V in the absence of phospholipids and/or in the presence or absence of Ca²⁺. The significance of these results as they might apply to the configuration of the prothrombinase complex and its interaction with prothrombin plus the usefulness of the short chain fatty phospholipid in exploring these lipid-protein interactions are discussed.